1. Field of the Invention
This invention relates to an apparatus and method for detecting magnetic fields, particularly detecting magnetic fields using giant magnetoresistance sensors, and specifically the use of giant magnetoresistance sensors to detect magnetic fields to perform non-destructive evaluation of electrically conductive components.
2. Description of Prior Art
A variety of methods currently are used to perform non-destructive evaluation (NDE) of anomalies and stresses in metal structures used in buildings, bridges, and aircraft components. Known techniques of NDE are based upon visual inspections, eddy current, X-ray radiography, ultrasonics, and acoustic emissions. These methods suffer from limitations in inspection capability, speed of inspection, and cost of inspection. Although they have some of the same limitations as known NDE methods, newer techniques such as holography, thermography, and shearography have shown some promise in laboratory testing. However, no known commercially viable inspection system based upon these more sophisticated technologies has yet been developed.
Giant magnetoresistance (GMR) is an effect characterized by large changes in resistance of certain types of materials in response to the presence of a magnetic field. With GMR, there is a "giant" change in resistance (.DELTA.R/R) response that is markedly greater in magnitude than that obtained by ordinary anisotropic magnetoresistance (AMR) effects. Generally, materials and components observed to exhibit giant magnetoresistance consist of multiple layers of very thin (roughly 20 Angstrom) ferromagnetic film alternated with similarly thin layers of non-magnetic conducting films, typically of copper, cobalt, nickel, iron and other metals. There have also been reports of GMR materials made using powders as starting materials.
There are known methods for exploiting the GMR effect to sense magnetic field changes. U.S. Pat. No. 5,465,185, for example, describes a type of sensor made with GMR material oriented in a particular way to form a so-called "spin valve" sensor. Spin valve sensors can be fabricated as shown in U.S. Pat. No. 5,561,368, to compensate for the offset inherent in these devices when DC measurements are made. In addition to spin valves, there are a number of different ways to make other GMR materials and sensors (i.e. tunnel junction and other named device structures) using a variety of materials and physical orientations. U.S. Pat. No. 5,565,236, describes a method for making a type of GMR sensor.
GMR sensors have been widely investigated for application in magnetic detection heads (for reading binary data stored on the disks) for hard disk drives. The great potential of GMR sensors as magnetoresistive heads stems from the very large maximum change in resistance (.DELTA.R/R as much as 10-20% in some systems) that they can exhibit in response to magnetic fields, as compared to the .DELTA.R/R of 2% typical of known magnetoresistant films employing the usual AMR effect. U.S. Pat. No. 5,442,508 describes a GMR sensor used as a reproduction head.
In general, GMR films are in a high resistance state when the magnetization in the GMR multilayer is predominantly antiparallel in adjacent magnetic layers, and can be then brought to a low resistance state by the action of an applied field which rotates the layers' magnetization into a predominantly parallel orientation roughly along the applied field direction. Consequently, GMR sensors generate a signal, based on a change in resistance, in response to a change in an external magnetic field.
One known application of GMR sensors is to measure linear displacement. U.S. Pat. No. 5,475,304 describes a GMR sensor and apparatus for measuring linear and angular displacement particularly for use in high precision machining applications.
A variety of sensors presently are used in eddy current systems for NDE of electrically conductive objects. Eddy current systems that use an excitation coil for inducing an alternating magnetic field in an object, and a pick-up coil for detecting the magnetic field induced, are widely used for detecting defects or anomalies in metallic materials. This kind of eddy current system detects a flaw by detecting the metal loss due to the flaw. The flaw or crack in the metal disrupts the normal eddy current fields. This disruption of eddy currents changes the magnetic fields, which change is detected by variations in the inductance of the pick-up coil. Conventional eddy current equipment, with cup core type probes, is intended for the detection of large cracks emanating from fastener holes in second or interior layers. Cup core probes interrogate the entire fastener hole during inspection and consequently are not very sensitive to the presence of small cracks.
U.S. Pat. No. 3,449,644 describes a magnetic reaction testing device and method of testing utilizing a semiconductor means for magnetic field sensing of an eddy-current-reaction magnetic field. The device is specific exclusively to pairs of Hall effect detectors. The relative field strength mode of operation requires placement of large sensors outside the excitation coil and detects the variation between at least one pair of angularly separated detectors.
U.S. Pat. No. 3,450,986 describes a magnetic reaction testing device and method of testing utilizing semiconductor means for magnetic field sensing of an eddy-current-reaction magnetic field. A single sensor absolute field detector, a variation of the device shown in U.S. Pat. No. 3,449,644, is described. The specific use of a Hall effect absolute field detector requires sensor placement outside the excitation coil.
U.S. Pat. No. 4,495,466 describes an eddy current test probe with circumferential segments and a method of testing material surrounding fastener holes. The disclosed detection method is common to known coil-based eddy current detectors, having output proportional to defect area divided by total excitation area. A multipole ferrite cup is used for magnetic field concentration around the hole under inspection. This sensor involves multiple, complex windings specific to a given hole size, and is extremely sensitive to field distortion due to off-center location errors.
U.S. Pat. No. 4,677,379 describes a process and device for detecting cracks in riveted joints using an eddy current probe. The disclosed device uses a four coil quadrature sensor configuration with synchronous detector to determine asymmetry in a uniform excitation field due to elongation along a crack. The size and optimum diameter of the device's sensor arrangement must be equal to rivet spacing, and the four coils must be precisely matched.
U.S. Pat. No. 5,399,968 describes an eddy current probe, having a body of high permeability supporting drive coil and plural sensors, very similar to the device of U.S. Pat. No. 4,495,466 with the addition of an outer winding to provide deeper field penetration, the possibility of a rectangular structure in addition to the simple pot core, and the possibility of Hall sensors at each pole. The disclosed device is extremely sensitive to field distortion due to off center location errors of the sensors.
U.S. Pat. No. 5,510,709 describes an eddy current surface inspection probe and method for aircraft fastener inspection. The device uses a circular array of coil pairs to produce a differential output signal. The largest output signal indicates the axis of the anomaly, and total output indicates the relative size of the defect. The size of the sensor array must be matched to the size of the rivet under inspection, making it extremely sensitive to field distortion due to off center location errors.
U.S. Pat. No. 4,916,392 describes a contactless current control sensor device for magnetoelectric crack detection. A movable sensor or a line of fixed sensors is used to measure the radial field variation around a conductor carrying a fixed test current. Anomalies in the radial field are then associated with disruptions in uniform current flow presumably due to cracks in the conductor. The disclosed device requires that the defective sample carry stable fixed test currents of 10 to 20,000 Amps, and variations in conductivity or conductor dimensions are indistinguishable from crack effects.
U.S. Pat. No. 5,298,858 teaches a device for non-destructive testing of electrically conductive materials. The device uses square wave excitation to generate wide band eddy current pulse echos, and synchronously gates a detection window to isolate echos from regions of interest. The device is directed to the use of a circular magnetic field for fixed radius circular scans, presumably for fasteners. Second, it is extremely sensitive to off-center sensor location errors; data must be time-shift corrected at each separate sample point to correct for off center errors.
U.S. Pat. No. 5,554,933 describes a polar coordinates sensor probe for testing material surrounding fastener holes using multipole ferrite cup detector essentially similar to the device of U.S. Pat. No. 4,495,466. The disclosed device uses a separate toroidal coil to produce a directed field component which sweeps around a ferrite cup, ostensibly increasing the relative field in the vicinity of a defect compared to the total applied field. A separate toroidal coil, much larger than the ferrite cup detector is used causing difficulty in producing a uniform rotating field.
An article written by William F. Arvin of Quantum Magnetics, Inc. and published in the Review of Progress in Quantitative Nondestructive Evaluation, vol. 15, Plenum Press, New York, 1996, describes magnetoresistive eddy current sensors for detecting deeply buried flaws. The article discusses magnetoresitors and SQUID (superconducting quantum interference device) sensors. It does not specifically discuss giant magnetoresistance sensors and makes no mention of sensor arrays.
Most of the foregoing references disclose devices employing coil-type sensors or Hall effect sensors, which are unlike magnetoresistors or giant magnetoresistors. Those that do mention the use of magnetoresistance refer to older types of devices that do not have the properties of giant magnetoresistance.
A need remains, thus, for an apparatus and method of performing NDE which provides rapid inspection capability, the ability to detect a variety of anomalies including corrosion, cracks, and stresses both at the surface and deep inside both simple and complex parts, can be fabricated in a high-density array and which is simple to use, eliminating the need of a highly trained, highly paid operator, particularly in the performance of maintenance or in-service inspections (as opposed to production quality inspection).
The present invention fills this unmet need by using a GMR sensor in the detection apparatus or method. The use of GMR sensors is not an obvious extension of other types of sensors including conventional magnetoresistors because, among other reasons, GMR sensors require special orientations in magnetic fields to exploit the effects of giant magnetoresistance and can be fabricated in high-density 2- and 3-dimensional arrays. Accordingly, the present invention makes novel use of specific types of differential detection and self-nulling and/or self-biasing operations.